Reinstate sub-tests

[bjlittle]

On the advice and recommendation of @ConorMacBride the tests.test_subtests have been temporarily disabled in #150.

I've created this issue as a reminder to reinstate the sub-tests.

This issue can also serve as a focus for dev discussion on how we best go about doing this.

For reference:

• See this comment
• PR Test against JSON summaries (and bugfixes) #134
• PR Add test for image shape mismatch and fix bug #145